PTC device and battery pack using the same

ABSTRACT

A PTC device which surely achieve effective space utilization while easily excluding the possibility of the formation of a short circuit. The PTC device includes a PTC component 3 comprising a first electrode 11, a second electrode 13, and a PTC element 9 disposed between these electrodes, a first electrically conductive lead 15 which is connected to the first electrode and able to connect with a button terminal of a battery, a second electrically conductive lead 18 which is connected to the second electrode, and an insulating element 23. The insulating element covers the inner periphery 7 of the PTC component, the second electrode and a portion of the second electrode and the second electrically conductive lead on the second electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a PTC device which can be used as acircuit protection device and a battery pack or assembly using such adevice.

2. Introduction to the Invention

The use, as a circuit protection device, of a PTC (positive temperaturecoefficient of resistance) device comprising a PTC composition which hasthe "positive temperature coefficient of resistance" property so as toprotect batteries and/or circuits from overcurrent conditions andovertemperature conditions therefrom is disclosed in, for example, U.S.Pat. Nos. 4,255,698 (Simon) and 4,973,936 (Dimpault-Darcy et al), andJapanese Utility Model Application No. 4-75287.

The PTC composition is one which gives a device comprising thecomposition (i.e. a PTC device) the following characteristics: thedevice is in a low resistance condition at a temperature during a normaloperation (depending on circuit conditions in which the device is used,for example from room temperature to 40° C.); on exposure to a hightemperature due to, for example, unusually large current resulting fromthe formation of a short circuit or excessive charging (depending oncircuit conditions in which the device is used, for example from 60 to130° C.), the device switches into an extremely high resistancecondition. Thus, when such a device is included in a circuit and anabnormal current passes through the circuit, the device comes into thehigher temperature condition and switches into the higher resistancecondition so that it decreases the current passing through the circuitand thus protects electric elements forming the circuit, parts connectedto batteries and so on.

Such a PTC composition is well known and no additional description isrequired. Detailed description can be seen in, for example, JapanesePatent Kokai Publication No. 55-78406. The disclosures of thesedocuments are incorporated herein by reference.

Battery packs, in which a plurality of batteries are contained, arecommonly used in electric or electronic equipment such as cameras, videocameras, tools, portable phones and portable computers. It is desirableto make such equipment as compact (i.e. lightweight and small) aspossible from a viewpoint of its portability. Thus, the battery packscontained in the equipment are preferably compact.

Abnormal current may pass through a circuit in such equipment due to theoccurrence of a failure such as the formation of a short circuit. When aPTC device is used for the protection of the circuit from failure,effective space occupation of the device within the equipment isrequired from a viewpoint of the compact equipment.

In order for the equipment to be compact, U.S. Pat. No. 4,973,936discloses placing the PTC device directly onto a button terminal of abattery. In this PTC device, a PTC component is in the shape of a diskto which a ring shaped insulation element is bonded using an adhesive,and a central portion of the insulation element is a hole which canreceive the button terminal of the battery. For such a PTC device, sincethe PTC component is located on the top of the button terminal of thebattery, a space is required in the battery pack to accommodate thethickness of the PTC component and to prevent the formation of a shortcircuit between the electrodes of the PTC component. Thus, the batterypack is required to have a sufficient space along a longitudinaldirection of the battery.

Japanese Utility Model Application No. 4-75287 discloses a PTC device asa circuit protection device which comprises an electrode having acap-shaped portion which can receive a button terminal of a battery, ahalf-ring-shaped electrode, and a PTC element sandwiched between theelectrodes. With such a PTC device, since a distance between a side ofthe cap shaped portion of the electrode and the half-ring-shapedelectrode is small, the formation of a short circuit is possible betweenthose electrodes. In order to prevent such formation, a positionalrelationship between the cap portion and the PTC element and between thecap portion and the other electrode should be precisely adjusted. Suchadjustment leads to an increase of production cost of the PTC device.

SUMMARY OF THE INVENTION

As described above, the circuit protection devices which have beenhitherto developed are not necessarily satisfactory, and they still havethe problems exemplified above. Thus, further improved circuitprotection devices are required. In particular, a circuit protectiondevice satisfying both the following is required: first, because of thesmall size of the device itself, there is a need to overcome the veryreal possibility of short circuits between the device electrodes or thedevice failing to function as a device; second, the device should beequivalent in effective space utilization to devices proposedheretofore.

It is, therefore, an object of the present invention to provide a PTCdevice which can be used as a circuit protection device and which easilyexcludes potential formation of a short circuit while ensuring theeffective space occupation, and also to provide a battery pack in whichsuch a device is used.

In the first aspect, the present invention provides a PTC device whichis suitable for attachment to a button terminal of a battery, saiddevice comprising

(A) a PTC component which (i) has an outer periphery and an innerperiphery which defines a hole in the center of the PTC component, and(ii) comprises

(1) a laminar PTC resistive element which is composed of a PTCelectrically conductive polymer composition (which is also referred toas a "PTC element"),

(2) a first laminar electrode which is attached to a first surface ofthe PTC resistive element (which is also referred to as a "firstelectrode"), and

(3) a second laminar electrode which is attached to a second oppositesurface of the PTC resistive element (which is also referred to as a"second electrode");

(B) a first electrically conductive lead which comprises

(1) a first part which is attached to the first laminar electrode, and

(2) a second part which at least partially closes the hole of the PTCcomponent;

(C) a second electrically conductive lead which comprises

(1) a first part which is attached to the second laminar electrode, and

(2) a second part which extends away from the outer periphery of the PTCcomponent; and

(D) an insulating layer which covers the inner periphery of the PTCcomponent and at least a portion of the first part of the secondelectrically conductive lead.

In the PTC device according to the present invention, the PTC componentis defined by a first main surface formed by the first (laminar)electrode, a second main surface formed by the second (laminar)electrode, the outer periphery and the inner periphery.

Thus, the PTC device according to the first aspect of the presentinvention can also be regarded as a PTC device comprising a PTCcomponent, a first electrically conductive lead and a secondelectrically conductive lead, which device is characterized in that

(A) the PTC component comprises a PTC element, a first electrode and asecond electrode in which

(1) the PTC component (a) has a through hole which is located in itscenter and (b) is defined by an inner periphery and an outer periphery,and a first main surface and a second main surface which are opposed toeach other,

(2) the first main surface is formed by the first electrode, and thesecond main surface is formed by the second electrode, and

(3) a PTC element comprises a PTC electrically conductive polymer, andis located between the first electrode and the second electrode,

(B) the first electrically conductive lead comprises a first part and asecond part in which

(1) the first part is connected to the first electrode, and

(2) the second part extends across an opening of the through hole sothat it at least partly closes the opening, and is able to electricallyconnect with a button terminal of a plus electrode of a battery,

(C) the second electrically conductive lead comprises a first part and asecond part in which

(1) the first part is connected to the second electrode, and

(2) the second part extends away from the PTC component, and is able toelectrically connect with another electrical component, and

(D) an insulating element covers the inner periphery of the PTCcomponent and at least a portion of the first part of the secondelectrically conductive lead.

In a second aspect, the present invention further provides a batterypack (or assembly) comprising the above PTC device and a battery inwhich the PTC device is placed on a button terminal side of the batteryso that the terminal is connected to the second part of the firstelectrically conductive lead.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of the PTC device accordingto the present invention;

FIG. 2 is a schematic plan view of the PTC component which is used forthe PTC device according to the present invention;

FIG. 3 is a schematic plan view of the PTC component including the firstconductive lead, which component is used for the PTC device according tothe present invention;

FIG. 4 is a schematic plan view of the PTC component including thesecond conductive lead, which component is used for the PTC deviceaccording to the present invention;

FIG. 5 is a schematic plan view of the PTC device according to theinvention;

FIG. 6 is a schematic view of the battery pack in which the PTC deviceaccording to the invention is used;

FIG. 7 is a schematic plan view, as shown in FIG. 3, of anotherembodiment of the first conductive lead which is suitably used for thePTC device according to the present invention;

FIG. 8 is a schematic plan view (as viewed from the top of FIG. 1) of asingle component in which the second conductive lead and the insulatingelement are made unified;

FIG. 9 is a schematic cross-sectional view (as viewed along a directionperpendicular to the plan view of FIG. 8) of a single component in whichthe second conductive lead and the insulating element are made unified;and

FIG. 10 is, similarly to FIG. 1, a schematic cross-sectional view ofanother embodiment of the PTC device according to the invention whereinthe insulating element and the second conductive lead form a singlecomponent as in FIG. 8 or 9.

In the above drawings, the reference numbers indicate as follows:

1,1' PTC device

3 PTC component

5 outer periphery

7 inner periphery

9 PTC element

11 first electrode

13 second electrode

15 first conductive lead

16 first part of first conductive lead

17 second part of first conductive lead

18 second conductive lead

19 first part of second conductive lead

21,21' second part of second conductive lead

23 insulating element

25 hole in first conductive lead

27,27' hole in PTC component

28 battery assembly

29 battery

31 button terminal

33 battery

35 button terminal

41 second mail surface

43 first main surface

45,47 minus electrode

51,53 locating tab

61 unified component

65 inner periphery of unified component

67 raised rim of insulating element 23

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, the "button terminal" substantially means a"protruding part of a plus side electrode of a battery." For example,the plus side electrode of the battery means an electrode on a plus sideof a commercially available dry battery, and a plus terminal of thatbattery is a terminal of the plus side electrode, and usually aprotruding part of the plus side electrode. "The second part of thesecond conductive lead extends away from the PTC component" means thatthe second part extends outwardly from the side (or edge) of the outerperiphery of the PTC component (i.e. extends away from the PTCcomponent). Further, "electrically connect" means to make a connectionso that current can flow through the connection.

In the present invention, the insulating element covers the innerperiphery of the PTC component so that the formation of a short circuitbetween the button terminal of the battery and the PTC element formingthe inner periphery of the PTC component is prevented. Optionally, whenthe second electrode forms a portion of the inner periphery of the PTCcomponent (in other words, for example, when the second electrode is solocated on the PTC element that an inner edge of the second electrodeforms an upper portion of the inner periphery, as shown in FIG. 1), theinsulating element prevents the formation of short circuits between thebutton terminal and the second electrode and between the first electrodeand the second electrode.

In a preferred embodiment, the insulating element covers at least aportion of an area of the second main surface on which area no secondconductive lead is placed, so that the second main surface is at leastpartially covered with the insulating element. Therefore, when thebutton terminal of the battery is located within the through hole andelectrically connected to the first electrode, it is ensured that noother part of the plus electrode of the battery than the button terminalelectrically contacts the second main surface, whereby the short circuitformation is prevented between the plus electrode and the second mainsurface. That is, the partial covering of the insulating elementfunctions as a spacer which spatially separates the plus electrode ofthe battery away from the second main surface by a certain distance.

In addition, the insulating element preferably covers the first part ofthe second conductive lead, so that no short circuit is formed betweenthe plus terminal of the battery and the first part of the secondconductive lead. With respect to the relationship between the buttonterminal of the battery and the first part of the second conductivelead, even if the first part of the second conductive lead is locatedjust adjacent to the inner periphery of the PTC component with an edgeof the first part facing the button terminal (namely, the edge of thefirst part extends up to the inner periphery), no electrical contact isformed between the button terminal and the first part because thecovering of the insulating element on the inner periphery functions as aspacer.

In one preferred embodiment of the PTC device according to the presentinvention, at least a portion of the area of the second main surfacewhich area has no second conductive lead thereon but has the covering ofthe insulating element is a portion of the second main surface whichportion is adjacent to the inner periphery of the PTC component and morepreferably a portion of the second main surface which is adjacent to awhole of the inner periphery, (thus the portion is in a ring form), andthe first part of the second conductive lead is adjacent or close to theouter periphery of the PTC component. Being "adjacent" means that theinsulated second main surface or the first part of the second conductivelead is in contact with the inner periphery or the outer periphery.Being "close" means that the first part of the second conductive lead isnot in contact with the outer periphery, but extends to the vicinity ofthe outer periphery. In such an embodiment, since the insulating elementcovers both an inner area and an outer area of the second main surface,the contact between the second main surface and the plus terminal of thebattery is more surely prevented.

In another preferred embodiment according to the present invention, atleast a portion of the area of the second main surface on which area nosecond conductive lead is located but the insulating element is placedis a whole of the area of the second main surface on which the secondconductive lead is not located. Thereby, electrical contact iscompletely prevented between the second main surface and the plusterminal of the battery.

In a further preferred embodiment, the insulating element further coversthe outer periphery of the PTC component. Thereby, electrical contact isprevented between the second part of the second conductive lead and thePTC component.

In another embodiment of the PTC component of the present invention, theinsulating element covers at least a portion of the second conductivelead so that the insulating element and the second conductive lead forma single component. The use of such a single component makes assemblingof the device easier because the number of parts to be assembled isreduced. In this embodiment, the first part of the second conductivelead may have a geometry which is substantially the same as that of thePTC component The first part of the second conductive lead maycompletely cover the second electrode. Alternatively, the first part ofthe second conductive lead may be slightly smaller than the secondelectrode or may be covered in part by the insulating element, so that aportion of the second electrode is in contact with the insulatingelement. In this embodiment, the insulating element preferably coverssubstantially all the surface of the first part of the second lead whichis not in contact with the second electrode.

In addition, the insulating element can cover a side of the first partof the second conductive lead so as to create a rim (protrusion) at aninner periphery of the first part of the second conductive lead whichcontacts and insulates the inner periphery of the PTC component. Thethickness (or height) of the rim is preferably the same as that of thePTC component, so that when the PTC component and the single componentincluding the second conductive lead are attached to each other, thethickness of a total of the insulating component, the first part of thesecond conductive lead and the PTC component is constant in any crosssection of the PTC device (see FIG. 10).

An additional advantage of the use of the single component in which theinsulating element and the second conductive lead are unified is thatcritical areas of the PTC component, e.g. the inner periphery, areinsulated, but the PTC component is not necessarily required to becompletely covered by the insulating element. For example, the outerperiphery of the PTC component need not be covered with insulatingelement. This means that upon the actuation of the PTC device, the PTCcomponent can expand more freely than when the outer periphery iscompletely covered by insulating element.

In another preferred embodiment, for example, for the purpose ofassembling the battery pack, the PTC device of the present invention hassuch a size that it does not extend radially beyond the periphery of theplus electrode of the battery when it is placed on the battery, asdescribed below.

The PTC devices according to the present invention as described abovecan be used as a circuit protection device in various electricalcircuits and battery packs.

In the second aspect, the present invention provides a battery pack (orassembly) comprising the PTC device described as above and a battery. Insuch a battery pack, the button terminal is placed within the throughhole of the PTC device while the terminal electrically connects with thesecond part of the first conductive lead.

The battery pack of the present invention may have one or morebatteries. If the battery pack has a plurality of batteries, they can beconnected in series or in parallel. In these cases, the electricalconnection between the batteries is achieved by the PTC device of thepresent invention, and the connection between the minus electrode of thebattery and the PTC device is achieved by the second part of the secondconductive lead. In another way, the second part of the secondconductive lead may electrically connect with another electricalcomponent constituting a circuit instead of the minus electrode of thebattery.

According to the present invention, since the insulating elementsurrounds the button terminal of the battery, it prevents electricalcontact of the button terminal with the second electrode and the PTCelement. It also prevents electrical contact between the second mainsurface of the PTC component and the button terminal and between thesecond conductive lead and the button terminal, so that no short circuitis formed. Also, since the hole of the PTC device receives the buttonterminal, the space formed by the protruding button terminal can beeffectively used. Thus, when the PTC device of this invention is usedfor the production of the battery pack, it can minimize the size of thebattery pack.

The present invention will be explained in detail by preferredembodiments of the circuit protection device (PTC device) according tothe present invention with reference to the accompanied drawings ofFIGS. 1 to 10.

FIG. 1 shows a schematic cross-sectional view of one embodiment of thepreferred PTC devices according to the present invention in which thePTC 1 device comprises the PTC component 3, the first conductive lead 15and the second conductive lead 18, and the insulating element 23. In theillustrated embodiment, the insulating component 23 covers the innerperiphery 7, the first part of the second conductive lead 19, the outerperiphery 5 and the whole of the second main surface except an area onwhich the second conductive lead 18 is placed. The PTC component 3comprises the first electrode 11 and the second electrode 13 and the PTCelement 9 sandwiched by these electrodes. In the illustrated embodiment,the two electrodes form the first main surface 43 and the second mainsurface 41 of the PTC component 3 which are opposed to each other.

FIG. 2 shows a schematic plan view (as viewing the embodiment shown inFIG. 1 from its top or bottom) of only the PTC component 3 whichconstitutes the PTC device 1 according to the present invention. As seenfrom FIGS. 1 and 2, the PTC component 3 is defined by the main surfaces41 and 43, the outer periphery 5 and the inner periphery 7. It is in theform of a disk having a through hole 27 in its center (thus, it is inthe form of a flattened ring shape as a whole of the component). Theinner periphery 7 defines the hole 27. The disk here does notnecessarily means a circle shape. As shown, a cross section of the outerperiphery 5 may not be a circle shape, and it may be, for example, of arectangular or a polygon, or it may be of a suitable combination of acircle, a rectangular and a polygon. The same is applied to a crosssection of the inner periphery 7. However, in one preferred embodiment,at least the inner periphery 7 has the cross section of a circle.

What is important is that the PTC device has such a structure afterbeing insulated by the insulating element that it can be easilypositioned onto the button terminal side of the battery and does notextends outwardly beyond the transverse cross section of the batteryexcept for the second part of the second conductive lead. It is alsoimportant that the hole 27 can receive the button terminal after thecovering of the insulating element has been applied. As long as this issatisfied, the PTC device 3 may be of any shape. For example, the PTCcomponent 3 is not necessarily in the closed ring form and may be of theopen ring form (namely, a ring form having a radial slit).

In the PTC component 3, the PTC element 9 to which the first electrode11 and the second electrode 13 are attached is made of an electricallyconductive polymer composition. The PTC component 3 is preferably madefrom a laminate of the PTC conductive polymer composition element whichis sandwiched between two metal foils as the electrodes.

The conductive polymer composition which can be used is well known forthose skilled in the art. Concretely, compositions described in thepatent publications which are referred to in the Introduction to theInvention section can be used, and such a composition comprises apolymer material and an electrically conductive filler, for example, inthe form of a particulate, which is dispersed in the polymer material.The polymer material preferably is an organic crystalline polymer, forexample, a polyolefin such as a polyethylene or an ethylene copolymer; afluoropolymer, such as a polyvinylidene fluoride; or a blend of two ormore of these polymers. The particulate conductive filler is preferablycarbon black, graphite, metal, metal oxide, glass coated with anelectrically conductive material, or a blend of these fillers. Theconductive polymer composition may contain other additives, e.g. anantioxidant, an inert agent, or a stabilizer, and may be crosslinked bya suitable manner (such as irradiation).

With respect to the resistivity of the conductive polymer composition,the resistivities during the normal operation and the unusual operationof the equipment in which the PTC device is used can be set as desiredby properly adjusting the kind and the amount of the conductive fillerto be used and the kind and the amount of the polymer material to beused depending on the equipment. For most applications, the conductivepolymer composition has, at a temperature of 20° C., a resistivity ofgenerally not more than 10 ohm-cm, preferably not more than 5 ohm-cm,and more preferably not more than 2 ohm-cm, and for example, a lowerresistivity such as not more than 1 ohm-cm is better.

The PTC element 9 is formed by a suitable manner, for example extrudingthe conductive polymer composition followed by cutting it, and attachingthe electrodes to have a sheet form. Both the first electrodes 11 andthe second electrodes 13 comprise an electrically conductive material,and are preferably metal in the form of a foil or a thin sheet, whichform is effective to reduce the volume of the device. As the metal, forexample, nickel, copper, or iron or an alloy such as brass or stainlesssteel may be preferably used.

In place of or in addition to the metal foil, either or both of theelectrodes may comprise a conductive paint or a graphite layer. It isparticularly preferred that, when metal foils are used for theelectrodes, they comprise an electrolyzed metal foil, which achievesgood adhesion between the PTC element and the electrodes. The attachmentbetween the electrodes and the PTC element of the conductive polymercomposition may be achieved by any suitable manner, for example, byusing an electrically conductive adhesive, by applying the electrodes tothe polymer composition when it melts or by pressure application.

In the PTC device 1 according to the present invention, the firstelectrode 11 of the PTC component 3 is electrically connected to thefirst conductive lead (or lead terminal) 15, and the second electrode 13is electrically connected to the second conductive lead (or leadterminal) 18. The connections may be made by any suitable manner, forexample, by means of soldering, welding, or using an electricallyconductive adhesive. In one preferred embodiment, merely pressing anappropriate electrode plate onto the conductive lead terminal issufficient.

FIG. 3 schematically shows, as viewed from the bottom in FIG. 1, the PTCcomponent 3 in which the first conductive lead 15 has been attached. Thefirst conductive lead 15 comprises a first part of the first conductivelead 16 which is attached to the first electrode 11 and a second part ofthe first conductive lead 17 which is located below the hole 27 of thePTC component 3. The first part 16 covers at least a portion of andpreferably substantially the whole (as shown in FIG. 3) of the surfaceof the first electrode 11. A portion of the second part 17 (PTCcomponent 3 side) of the first conductive lead 15 is able to be directlyconnected electrically to the button terminal of the battery by means ofa suitable manner, for example, soldering, applying a pressure orwelding.

The second part 17 covers at least a portion of the opening of the hole27 of the PTC component 3. Optionally, the second part 17 may have oneor more openings 25. As seen from the comparison between FIGS. 2 and 3,the shape of the outer periphery of the first conductive lead 15 may besubstantially the same as that of the PTC component 3.

FIG. 4 shows, in a schematic plan view as viewed from the top in FIG. 1,the PTC component 3 to which the second conductive lead (or leadterminal) 18 has been attached. The second conductive lead 18 comprisesa first part of the second conductive lead 19 and a second part of thesecond conductive lead 21. The first part 19 is apart which is attachedto at least a portion of the second electrode 13. The first part 19 maybe attached to cover the whole of the second electrode 13, but it isusually sufficient that the first part 19 partly covers the secondelectrode 13 as shown for easily manufacturing the device. One preferredshape of the first part 19 may be a part of the flat ring shape as shown(i.e. an arc shape) or a "U" shape.

The second part 21 of the second conductive lead 18 comprises a partwhich extends away from outer periphery 5 of the PTC component. Thesecond part 21 functions to make electrical contact with another batteryor other electric part, and it may be bent as shown in FIG. 1, ifnecessary.

The first and second conductive leads 15 and 18 can be made from anysuitable material such as a metal or an alloy (e.g. nickel, stainlesssteel, copper, iron, brass or bronze). For the second conductive lead18, a lead frame may be used in the production process.

FIG. 5 schematically shows a plan view of the PTC device 1 as viewingthe device from the top in FIG. 1; namely, shows a condition in whichthe insulating element 23 has been applied to the embodiment shown inFIG. 4. In the PTC device 1 according to the present invention, theinsulating element 23 covers the inner periphery 7 of the PTC component3, at least a portion of an exposed area (i.e. an area which is notcovered with the second conductive lead) of the second electrode 13, andthe first part 19 of the second conductive lead 18. With such anarrangement of the insulating element, when the button terminal of theplus electrode of the battery is placed within the hole 27', noelectrical contact is formed between the plus electrode and both thesecond main surface and the first part of the second conductive lead,and also between the button terminal and the PTC component so that noshort circuit is formed between them.

In a preferred embodiment, as shown in the figures, the insulatingelement 23 covers the inner periphery 7 of the PTC component 3,substantially the whole of the exposed area of the second electrode 13and substantially the whole of the first part 19 of the secondconductive lead 18. In addition to the preferred embodiment, it is morepreferable that the insulating element covers at least a portion of thesecond part 18 adjacent to the first part 19. Further, in anotherembodiment, the insulating element 23 may completely cover the outerperiphery 5 in addition to the more preferred embodiment, so that thePTC element 9 on the first conductive lead is prevented from beingcontact with the second part 21 of the second conductive lead 18. Withthis arrangement, the formation of the short circuit between the buttonterminal of the battery and the second electrode is prevented.

The thickness of the insulating element 23 is sufficient to ensure thatthe insulating element 23 resists abrasion which is experienced duringthe usual use of the device, and especially to ensure that there isformed no short circuit between the button terminal of the battery andthe PTC component 9 or the second electrode, but is thin enough tominimize the space required when the device is positioned on the buttonterminal. The dimensions of the PTC device as a whole are determined soas to ensure installation of the device onto the button terminal of thebattery.

The insulating element 23 preferably comprises an insulating material.As such a material, a polymer such as a polyamide, a polybutyleneterephthalate (PBT), a polyester, a polyethylene, a polyvinylidenefluoride, or an epoxy resin may be used. When the insulating material isa thermoplastic material, it has a melting point which is higher than amelting point T_(m) of the conductive polymer composition (thus, amelting point of the polymer which constitutes the composition) so as toensure that substantial deformation of the insulating element is avoidedwhen the PTC device is actuated and its temperature is raised.

Generally, it is preferred that the insulating element softens at atemperature greater than 130° C. in order to provide rigidity during itsnormal use. In addition, it is desired that the coefficient of thermalexpansion of the insulating component is not so different from that ofthe conductive polymer composition. Preferably, those coefficients aresubstantially the same.

The insulating element can be applied to the PTC component 3 in thecondition as shown in FIG. 4 by any suitable means, e.g. injectionmolding, spray coating, or dipping. In another embodiment, a moldedarticle of the insulating element is formed beforehand by extrusion andthen the article is applied to the PTC component 3 in the embodiment asshown in FIG. 4 by any suitable manner (e.g. using an adhesive orfusing). Optionally, an additional step may be carried out, for example,a curing step of the polymer material by heat, irradiation orultraviolet light as required. Those forming methods have advantagesthat they are readily carried out, the insulating element is easilyarranged and they are cost-effective. It is also preferred to use amethod, for example, injection molding, which has little thermal effecton the conductive polymer composition of the PTC element.

The PTC device 1 according to the present invention is made by forexample the following process: the PTC component 3 in the form of a ringis stamped out from a laminate comprising the conductive polymercomposition 9 which is sandwiched between the two metal foils (11 and13); then the first and second conductive leads (15 and 18) are attachedto the PTC component, in a preferred embodiment, by applying solderpaste between the first part 16 of the first conductive lead 15 and thefirst electrode 11, and also between the first part 19 of the secondconductive lead and the second electrode 13 and reflowing the solderpaste by means of hot air or in an oven followed by re-solidifying thepaste; and finally the insulating element 23 is applied by means ofinjection molding. A diameter of the hole 27' which is formed after theinsulating element 23 is applied is smaller than that of the originalhole 27, and it is such that the button terminal of the battery isreceived within the hole 27' and the terminal is in contact with thesecond part of the first conductive lead 15 (see FIG. 6).

The resistance of the PTC device may be adjusted depending on itsintended application, i.e. equipment in which the device is used. Theresistance at 20° C. is generally not more than 1 ohm, preferably notmore than 0.5 ohms, and more preferably not more than 0.25 ohms, e.g.0.050 to 0.15 ohms.

The PTC device 1 of the invention is particularly suitable for the usein a battery pack or assembly 28 as schematically shown in FIG. 6. Thebattery pack 28 comprises the batteries 29 and 33 which comprise buttonterminals 31 and 35 respectively, and the PTC circuit protection devices1 and 1' according to the present invention.

The PTC device 1 is so positioned that the hole 27' receives the plusterminal 31 to make electrical contact between the second part 17 of thefirst conductive lead 15 and the button terminal 31, and that the secondpart, and preferably the end of the second conductive lead 18, isconnected to the minus terminal 45 of the adjacent battery 33. Inanother PTC device 1' of this invention, the second conductive lead 21'can be attached to a minus terminal surface of a battery (not shown) orother electrical part of the circuit (not shown). The minus terminal 47of the battery 29 may be connected to an electrical component in acircuit.

As seen from FIGS. 1 and 6, when the circuit protection device accordingto the present invention is used, the space in the battery pack may bemost effectively used. That is, the PTC device 1 only slightly extendsthe length of the battery pack along a height direction of the buttonterminal of the battery by the thickness of the first conductive lead15. It is to be noted that the drawing in any Figure is shownexaggeratedly (especially in the thickness direction) for the ease ofunderstanding. (Particularly, in FIG. 6, although it appears that thelength along the longitudinal (or height) direction of the battery isconsiderably enlarged, the distance between an outermost surface of thelead terminal 15 of the PTC device and the end surface of the plusterminal 31 is substantially equivalent to the thickness of the leadterminal 15.)

The battery 29 to which the present device is applied may be any type ofthe battery, either a primary battery or a secondary battery, and forexample a nickel-cadmium, nickel-hydrogen, alkaline, or lithium batterymay be used. Particularly, it is preferred to use the device with a"AAA" type battery.

FIG. 7 illustrates another embodiment of the first lead which issuitable for use in the PTC device of the present invention in aschematic plan view as in FIG. 3. In this embodiment, one or morelocating tabs 51 and 53 (two tabs in the illustrated embodiment) areprovided at the outer periphery of the first conductive lead 15 to allowfor precise and efficient assembly, for example, when automaticassembling equipment is used. Similar locating tab(s) may be provided onthe second conductive lead.

FIGS. 8 and 9 show, in a plan view (namely, as viewed from the bottom inFIG. 1), a single component 61 in which the second conductive lead 18and the insulating element 23 are formed integrally. In this embodiment,the first part 19 of the second conductive lead is in the form of a disk19 which has the second part 21 extending away from the first part.Insulating element 23 comprises an outer periphery 63 and an innerperiphery 65, and covers substantially all of one surface of the firstpart 19. The inner periphery 65 has a raised rim 67 of the insulatingelement which is designed to completely contact the inner periphery 7 ofthe PTC component 3.

FIG. 10 shows in cross-section the PTC device 1 in which the singlecomponent and insulating component and the second conductive lead areformed into the single component 61 as shown in FIG. 8 or 9.

As seen from the above, the present invention provides a PTC devicewhich will not suffer from the formation of short circuits by preventingdirect contact between the button terminal of the battery and, forexample the PTC element, the second conductive lead and the secondelectrode of the PTC component by means of the insulating element. Sincethe PTC device of the present invention can most effectively use thespace around the button terminal of the battery when the device is usedfor the battery pack, the battery pack is not substantially enlarged insize.

The PTC device of the present invention is not disassembled duringshipment or an assembly process of the battery pack when the insulatingelement is made unified with the PTC device. In addition, the assemblingprocess of the battery pack can be simple, and for example preciselocation of the PTC device is easily carried out through the insulatingelement when the PTC device is placed onto the battery, which makes theprocess not complicated.

What is claimed is:
 1. A PTC device suitable for attachment to a buttonterminal of a battery, said device comprising(A) a PTC component which(i) has an outer periphery and an inner periphery which defines a holein the center of the PTC component, and (ii) comprises(1) a laminar PTCresistive element which is composed of a PTC electrically conductivepolymer composition, (2) a first laminar electrode which is attached toa first surface of the PTC resistive element, and (3) a second laminarelectrode which is attached to a second opposite surface of the PTCresistive element; (B) a first electrically conductive lead whichcomprises(1) a first part which is attached to the first laminarelectrode, and (2) a second part which at least partially closes thehole of the PTC component; (C) a second electrically conductive leadwhich comprises(1) a first part which is attached to the second laminarelectrode, and (2) a second part which extends away from the outerperiphery of the PTC component; and (D) an insulating layer which coversthe inner periphery of the PTC component and at least a portion of thefirst part of the second electrically conductive lead.
 2. The deviceaccording to claim 1 wherein the insulating element covers at least aportion of the second laminar electrode.
 3. The device according toclaim 2 wherein the insulating element covers the inner and outerperipheries and the whole of the second laminar electrode.
 4. The deviceaccording to claim 2 wherein the insulating element and the secondelectrically conductive lead form a single component.
 5. The deviceaccording to claim 1 wherein the insulating element covers the inner andouter peripheries and the whole of the second laminar electrode.
 6. Thedevice according to claim 1 wherein at least the first electricallyconductive lead comprises at least one locating tab.
 7. The deviceaccording to claim 1 wherein the first and second electricallyconductive leads comprise nickel, stainless steel, copper, iron or analloy.
 8. The device according to claim 1 wherein the insulating elementand the second electrically conductive lead form a single component. 9.The device according to claim 1 wherein the insulating element is apolymer which is made of a polyamide, a polyester, a polybutyleneterephthalate, a polyethylene, a polyvinylidene fluoride, or an epoxyresin.
 10. A battery assembly which comprises(A) a battery whichcomprises a button terminal; and (B) a PTC device which comprises(1) aPTC component which (i) has an outer periphery and an inner peripherywhich defines a hole in the center of the PTC component, and (ii)comprises(a) a laminar PTC resistive element which is composed of a PTCelectrically conductive polymer composition, (b) a first laminarelectrode which is attached to a first surface of the PTC resistiveelement, and (c) a second laminar electrode which is attached to asecond opposite surface of the PTC resistive element; (2) a firstelectrically conductive lead which comprises(a) a first tart which isattached to the first laminar electrode, and (b) a second part which atleast partially closes the hole of the PTC component; (3) a secondelectrically conductive lead which comprises(a) a first part which isattached to the second laminar electrode, and (b) a second part whichextends away from the outer periphery of the PTC component; and (4) aninsulating layer which covers the inner periphery of the PTC componentand at least a portion of the first part of the second electricallyconductive lead,and which is placed over the button terminal so that thesecond part of the first electrically conductive lead is in physical andelectrical contact with the button terminal.
 11. The assembly accordingto claim 10 wherein the battery is a nickel-cadmium battery, anickel-hydrogen battery or a lithium battery.
 12. The assembly accordingto claim 11 which comprises at least two batteries and wherein thesecond part of the second electrically conductive lead extends from onebattery to another battery to electrically connect between thebatteries.
 13. The assembly according to claim 10 which comprises atleast two batteries and wherein the second part of the secondelectrically conductive lead extends from one battery to another batteryto electrically connect between the batteries.
 14. The assemblyaccording to claim 10 wherein the battery is a AAA battery.
 15. Thedevice according to claim 1 wherein the insulating polymer softens at atemperature higher than 130° C.
 16. The device according to claim 1wherein the electrically conductive polymer composition has a meltingpoint T_(m) and the insulating polymer is a thermoplastic material whichhas a melting point higher than T_(m).